Mooring systems for floating structures have been proposed which include a plurality of mooring lines anchored to the bottom and connected at their upper ends to the floating structure and extending in substantially parallel relationship to each other with the buoyancy of the floating structure being used to maintain the mooring lines under a preselected tension. In this form of mooring system, the floating structure is held substantially level and horizontal movement away from a preselected site introduces forces in the mooring lines urging the structure toward its equilibrium position.
It has been suggested to utilize templates on the bottom for multiple drilling and for making production connections. Also, it is well known to have guide lines extending from such drilling template and landing bases to the surface of the water to allow equipment to be lowered into proper position on the template. Still further it has been previously known that floats may be secured to guide lines at a point below the water surface to allow a vessel to disconnect from the lines and later relocate the lines by sonar and manually recover them. Others have further suggested that the use of buoyant members to support mooring lines at intermediate positions below the surface of the water. Additionally others have disclosed a buoy or a riser pipe and a line connecting to a secondary buoy or the surface to allow disconnecting from the riser pipe during a storm and relocation of the surface buoy.
Still further it has been disclosed that a mooring cable may be connected to a buoy and a rope or steel cable connecting from the buoy to the floating structure. The buoy surrounds and maintains the upper end of the mooring cable at or near the water surface.
It has been previously suggested that the anchor system for a tension moored drilling and production platform could be set by the platform when it is delivered to the drilling site. This notion is best applied when gravity anchors are to be used instead of piling and has been previously disclosed in a Ray U.S. Pat. No. 3,919,957, of common assignment with the instant patent. However, gravity anchors are large and heavy structures which represent additional complication and expense in the fabrication of a platform in order that it be capable of setting such structures. This complication is worthwhile for a mobile platform which is intended to be moved frequently during its lifetime. In the case of a more permanent installation, however, it is more economical and timely to utilize anchor piling which can be placed while the drilling or production platform is being constructed. Thus, the anchorage system is ready for connection to the platform when said platform is delivered to the intended site. In this manner valuable time is saved in platform installation and drilling and production of a petroleum reservoir can begin more quickly.
Corrosion of mooring lines is a primary consideration in the use of tension moored platforms because the mooring lines are primary structural elements under considerable tension. Failure of the mooring lines may represent a costly delay to the drilling and production activities and may even result in damage to the floating platform. Such a failure may result from loss of strength of the mooring lines as corrosion reduces their available cross sectional area. It is well known that the most corrosive environment for mooring cables is near the interface between sea and air, commonly referred to as the splash zone. Here water is highly oxygenated from mixing due to wind and wave action. Furthermore, constant wave action erodes the oxides away from the metallic surface exposing new metal to be attacked by corrosion. Protective coatings can be used on mooring lines within the splash zone as long as said lines require no handling or manipulation. Where handling is required, however, the coating is soon destroyed which renders the remaining coating essentially useless.